Process for removing acid gases from gascous method

ABSTRACT

When using potash or other alkali metal salts such as carbonates, hydroxides, hydrosulfides, sulfides, or bicarbonates to absorb acid gases such as H2S and CO2 from gaseous mixtures, unexpectedly high absorption rates are obtained by adding amines which when present in critical amounts exhibit regions of liquidliquid-gas immiscibility in the separation system.

United States Patent Leder [15] 3,637,345 [451 Jan. 25, 1972 [54]PROCESS FOR REMOVING ACID GASES FROM GASCOUS METHOD 211 Appl. No.:825,444

I 52] U.S. Cl. ..23/2 R [51] Int. Cl. 5 8] Field of Search [56]References Cited UNITED STATES PATENTS 1,882,289 10/1932 Lush ..23/2 APerkins, ..23/2 A 1,951,992 3/1934 2,161,663 6/1939 Baehr et al. ..23/3LA 2,718,454 9/1955 Wylie ..23/3 X Primary Examiner Earl C. ThomasAttorney-Chasm and Sinnock and Michael Conner [57] ABSTRACT When usingpotash or other alkali metal salts such as carbonates, hydroxides,hydrosulfides, sulfides, or bicarbonates to absorb acid gases such asH,S and CO, from gaseous mixtures, unexpectedly high absorption ratesare obtained by adding amines which when present in critical amountsexhibit regions of liquid-liquid-gas immiscibility in the separationsystem.

12 Claims, No Drawings PRIOR ART The use of potash or other alkali metalsalts ,toabsorb acid gases such as CO, and H 8 is well known. In US.Pat. No. 2,7l8,454 Patentee makes use of potashandsimilaralkali metalsalts in conjunction withamines such as monoethanol gases from asolution. The combination of the alkali metal compounds in conjunctionwith the amine yields higher capacity for acid gases than systems withthe amine alone.

In British Pat. No. 1,063,517 to Allen G. Eickmeyer the problem ofremoving acid bodies such as CO, and.I-I,S is again faced. Patenteeteaches the use of potassium carbonate and A other alkali metalcarbonates in conjunction with particular amines which will avoidcorrosion problems and at the same time greatly accelerate theabsorption and subsequent desorption of the CO and H 8. Specifically,patentee makes use of ethylene polyamines, alkanolamines or alkanolamine.borates as well as mixtures thereof. Examples of such .amines areethylene diamine, diethylene triamine, diethanolamine, ethylenediamine,etc.

All of these techniques represent a method by which some of the acid gasmay be removed; they have, however, shown a limited rate and capacityfor acid body removal, relative to this invention.

SUMMARY OF THE INVENTION According to this invention it has unexpectedlybeen discovered that the use of particular amines such as piperidine,1,6-hexane diamine, their derivatives and other organic amines whosecarbon to nitrogen ratios vary between lzl and 6:l, when added tosolutions of alkali metal salts, in critical amounts, tend to causeareas of phase separation. When acid gas containing gaseous mixtures arecontacted with these two phase systems, the removal of the acid gas isseveral times greater than the removal which was effected in previousalkali metal amine systems in which only one phase existed.

This phase separation takes the form of alower-criticalsolution-temperature type of behavior. Solubility limitsdepend on solid concentration, CO content of the mixture andtemperature. The incipient phase separation or actual occurrence of twoliquid phases tends to destabilize the liquid interface causing localvariations in concentration and surface tension and rapid surfaceremoval thereby facilitating mass transfer.

The contacting of the absorbent mixture and the acid gas may take placein any suitable contacting tower. Acid gas includes sulfur dioxide,carbon dioxide, and oxides and sulfur derivatives of C, through C,hydrocarbons. These acid gases may be present in trace amounts within agaseous mixture or in major proportions.

The absorbing solution will comprise a major proportion of alkali metalcompounds and a minor proportion of the amine. The amount of aminenecessary to form two phases will vary with conditions such astemperature, alkali metal salt content,

acid gas content and the particular amine. For piperidine and itsderivatives and 1,6-hexane diamine and its derivatives, at a temperatureof 50 to 150 C. and a salt content of to 25 weight percent, region ofimmiscibility will first form at a concentration of amine above about 4weight percent. Preferred amine concentrations are 4 to about 20 weightpercent. Acid gas content, to facilitate the formation of the separatephases should be less than 50 percent of saturation, preferably lessthan 30 percent.

The absorbent mixture, after absorbing the acidic bodies is thensubjected to regeneration. In the regeneration unit, the liquid is closeto saturation with acid gas and is therefore one phase. Stripping ofacid gas may therefore be accomplished conventionally by blowing withsteam or inert gas according to techniques well known in the art.

.More specifically, the instant invention pertains to a method forremoval of acid gases from gaseous mixtures in which they are carried.By acid gases it is meant CO H S, oxidation productsand sulfides ofmethane, ethane, propane, and butane. Such acid, gases in the fluid orgaseous state are found in mixtures of naturalgas, hydrogen, andhydrogen and nitrogen.

. The removal of these acid gases is essential since some of them ..are-.inherently corrosive, some tend to precipitate in amine,diethanolamine and triethanolamine to remove acid Y processing, some arepollutants and some catalytic poisons. Toremove'these acidgases'from themixture, the mixture,

.-while in a fluid, preferably gaseous state, must be'brought intocontact with asuitableabsorbent which will preferentially ab- ,sorbtheacid gases. and atthe same timesubstantially exclude other elementsof the mixture. It has been knownin the past to use alkali metalcarbonates, bicarbonates, hydroxides,

hydrosulfides, and sulfides for the absorption of the acidic bodiesfroma fluid orgaseous mixture. Typical'compounds v.which have been utilizedincludesodium hydroxide, sodium .hydrosulfide, sodium sulfide, sodiumbicarbonate, sodium carbonate, lithium hydroxide, lithium hydrosulfide,lithium sulfide, lithium bicarbonate, lithium carbonate, potassiumhydroxide, potassium hydrosulfide, potassium bicarbonate and potassiumcarbonate.

The preferredalkali metal salts are potassium carbonate .and. sodiumcarbonate. The alkali metal salts, i.e., carbonates .are present in anamount of about 5 to 25 weight percent of .thesolution. It should benoted that the solution is preferably aqueous. A preferred amount ofalkali metal salt would be from 10 to 20 percent by weight'and mostpreferred range of saltis approximately 15 to 20 weight percent.

The absorptive ability of the alkali metal compound is greatly increasedby adding amines which have the capacity to form, under specifiedconditions, an immiscible region within the alkali metal solution.Absorption rates are increased from two to five times above-those foundwhen one utilizes the previously mentioned techniques for the removal ofacid gases.

The amine which is tobe utilized in the instant invention should have acarbon-to-nitrogen ratio between Hand 6:] and when added in criticalproportions under appropriate conditions will-form thenecessary regionsof immiscibility. To accomplish this, an amine concentration of aboveabout 4 weight percent should be utilized. Preferable range of aminepresent within the aqueous solution is about 5 to 20 weight percent andmost preferably 8 to 12 weight percent. These ratios will, of course, beapplicable to the typical absorption temperatures which arein-the regionof about 50 to l50 C. For different temperatures, different ratios willhave to be calculated. Additionally, salt content will be between about5 to 25 weight percent of the aqueous solution. The acid gas con- .tentof the solution, to maintain two phases, should be less than about 50percent of saturation, preferably less than about 30 percent ofsaturation. Should the acid gas concentration increase to substantiallymore than 50 percent of saturation the two phases will disappear.

Preferred amines to be used under the conditions outlined above arepiperidine and its derivatives. By derivatives it is meant thefollowing:

wherein R, to R are selected from the group consisting of hydrogen, C,to C alcohols, C, to C alkyl groups, C, to C, alkenyl groups, aminogroups, carboxyl groups, hydroxyl groups, sulfides, .hydrosulfides, andphosphate groups. It is preferred that hydrogen and C, to C alkyl oralkenyl groups are utilized. Most preferred is that the various Rs aremethane and hydrogen in which case piperidine itself is utilized.

tives thereof. By derivatives of l,6-hexane diamine it is meant thefollowing:

The contacting between the acid gas containing fluid mixture and theabsorptive aqueous solution may be cocurrent or countercurrent;countercurrent is preferred. Temperature during the contacting may varybetween 50 and l50 C., preferably 90 to 120 C. and most preferably 100to l l C. Pressures may vary widely, between 0 p.s.i.g. and 1,000p.s.i.g., preferably 100 and 500 p.s.i.g., and most preferably 300 and400 p.s.i.g. Generally, the countercurrent contacting to remove the acidgas will, last for a period of from I to 60' minutes. The contacting maytake place when the acid gas containing mixture is either in the liquidor gaseous state but Measured across u 410 cm. interface 80' C. I50r.p.m. in 3,800 cc, of 20% Potash buffer in which the HCOJCO, ratio is I:l

Above about 5 weight percent the pipen'dine. l,b-hcxamethylenediamineformed two phases. As the CO, concentration increased the second phasetended to disappear.

Below 5 percent amine all the various solutions were totally miscible.Above about 5 weight percent piperidine and 1,6- hexamethylenediaminethe partial miscibility effect was observed and the absorption rateswere significantly higher as may be seen from the table. At this pointthe piperidine absorption rate was 2.70 moles per hour as compared to1.20 for the diethanolamine borate. This is a significant difference; at18 percent amine the piperidine rate is some 60 percent higher than thatobserved with ethanolaminoethanol. EXAMPLE 2 ln this example the exactconditions of example I are utilized except that instead of thecompounds described in example l the gaseous state is much preferred.The contacting zone may,;.

be a packed tower, sieve trays or any convenient gas scrubber.

A standard stainless steel contacting tower is partieularlyeffective.The tower may be equipped with bubble ,tr ays, Raschig rings many otherwell-known packing material to facilitate contacting. The gaseousmixture which is recovered with substantially all of the acidic gasesremoved may be used for many purposes, i.e., ammonia synthesis,hydrotreating or liquefied natural gas production. Absorbing solution,which is saturated with acidic gases such as CO and H 8 must then beregenerated so that it may be used again. It is regenerated byconventional means; typically, steam is passed through the liquid andacid gases are stripped out. The absorbing solution after being cleansedof the acidic bodies, may be recycled back to the contacting tower.Makeup adsorbent may be added as needed. Alternative methods forregenerating the contaminated adsorbent include air or N stripping.

SPECIFIC EMBODIMENTS EXAMPLE I In these examples, runs were madecomparing various additives in a potassium carbonate aqueous solution.Four different amines were compared, at different concentrations.Piperidine, ethylaminoethanol, diethanolamine borate, and

l,6-hexane diamine were the compounds. All of these com pounds were usedto remove CO from a pure gas phase. The CO, was contactedcountercurrently in the gaseous phase with the various absorbentsolutions. The contacting took place in a stirred vessel of 9-inchdiameter which contained about 4 liters of absorbent solution. Asindicated in the table the weight of the various amines was varied from0 to 18 weight percent. Contacting took place at a temperature of 80 C.and a pressure of 780 mm. for a period of up to about 3 hours.

TABLE initial Absorption Rates Rate (moles/hr. Diethanolaminc Wt. "kPipcridine EAE Boratc l-fiHDA 0 0.7!) 0.70 0.70 2.5 1.70 1.60 L00 5 2.701.10 1.20 L80 l0 3 2. 20 L40 2.5

is utilized. The concentration of this compound is varied to correspondto the concentration of piperidine in example I. Substantially identicalresults are achieved, i.e., at a concentration of 5 weight percent 2.2moles per hour of CO are absorbed. EXAMPLE 3 In this example the exactconditions of example 1 are utilized again except that in place of thecompounds described in example 1 is utilized. Again resultssubstantially identical to those achieved with piperidine in example 1are found. EXAMPLE 4 In this example conditions identical to example 1are utilized except that is substituted for the compounds in example 1.The results achieved are substantially identical to those achieved inexample I when utilizing piperidine. EXAMPLE 5 In this example the exactconditions of example i are utilized except that is substituted for thecompounds in example I. Here again results substantially identical tothose achieved for piperidine are obtained.

EXAM PLE 6 In this example the exact conditions of example I areutilized except that is substitutedforthe compounds in example I.Results substantially identical to those achieved in example. i with1,6- hexane diamine are observed.

EXAMPLE 7 in this example the exact conditions of example I are utilizedexcept that is substituted for the compounds. Results substantiallyidentical to those achieved with l,6-hexadiamine are observed.

What is claimed is:

l. A process for removing acid gases from a gaseous mixture, said acidgases selected from the group consisting of C0,, H,S, and oxidationproducts and sulfides of methane,

ethane, propane and butane which comprises contacting said amines havingthe general formula wherein R, to R are independently selected from thegroup consisting of hydrogen, C to C; alcohols, C to C alkyl groups, C,to C alkenyl groups, amino groups, carboxyl groups, and hydroxyl groups,at a temperature of 50 to 150 C., said amines being capable of fon'ningregions of immiscibility within said solution.

2. The process of claim I wherein said solution is no more than about 50percent saturated with said acid gases.

3. The process of claim 2 wherein said acid gas is C0,.

4. The process of claim 2 wherein said acid gas is H,S.

5. The process of claim 1 wherein said amine is piperidine.

6. The process of claim I wherein said amine is l,6-hexanediamine.

mixture with a solution, said solution comprising 5 to 25 weight percentof an alkali metal salt, said salt selected from the group consisting ofalkali metalcarbonates, bicarbonates, hydroxides, hydrosulfides andsulfides and at least about 4 weight percent of an amine, said aminebeing selected from the group consisting of amines having the generalformula 7. The process of claim [wherein said amine is .present in g theamount of 5 to 20 weight percent.

2. The process of claim 1 wherein said solution is no more than about 50percent saturated with said acid gases.
 3. The process of claim 2wherein said acid gas is CO2.
 4. The process of claim 2 wherein saidacid gas is H2S.
 5. The process of claim 1 wherein said amine ispiperidine.
 6. The process of claim 1 wherein said amine is1,6-hexanediamine.
 7. The process of claim 1 wherein said amine ispresent in the amount of 5 to 20 weight percent.
 8. The process of claim7 wherein said alkali metal salt is an alkali metal carbonate.
 9. Theprocess of claim 7 wherein said alkali metal salt is sodium carbonate.10. The process of claim 7 wherein said mixture is natural gas.
 11. Theprocess of claim 7 wherein said amine is piperidine.
 12. The process ofclaim 7 wherein said amine is 1,6-hexamethyldiamine.